So we've changed the number of partitions from 4 to 6. There are still 11 rows, but the information in information_schema.partitions seems to be wrong.
This is because innodb_stats_persistent is enabled.

The statistics can be updated by running 'ANALYZE TABLE pfoo'. Or by just waiting a few seconds as InnoDB will update the statistics automatically if innodb_stats_auto_recalc is enabled, but this is not instantaneous.